Not a foolproof way but visit a website like loco2.com
I input a journey from Paris to Le Mans on 26 Feb
Click on the price box - which is the "public fare" and quite often an Advance ticket.
Choose a Fully Flexible ticket and then discount this according to your FIP discount.
So, the 0806 shows a price of £37. If you click this price is shows that it is semi-flexible. The drop down box shows a £50 fully flexible fare. So FIP would either be £25 (with a 50% discount) or £12.50 (with a 75% discount).
The 0906 shows a price of £27.50. If you click this price it already shows a fully flexible ticket so just discount this price.
The above tends not to work in Spain as most trains are "Globally Priced" for FIP travel but I have found it to be pretty accurate for France, Netherlands, Germany, Belgium, and Switzerland and Austria.
